1. Field of the Invention
The invention relates to a comparator, and more particularly to a comparator with transition threshold tracking capability.
2. Description of the Related Art
On-chip oscillators are in high demand as an external crystal replacement by low-cost and single-chip systems. The frequency stability of an oscillator against variations in temperature and supply voltage is a critical specification for an on-chip oscillator. Working with wide range of supply voltages is also preferred, to gain the most operating time when powered by a battery.
In general, a wide supply range oscillator with typical frequency stability can better accommodate temperature changes. Furthermore, relaxation oscillators with voltage averaging feedback (VAF) or an offset cancellation technique have good temperature stability, wherein the relaxation oscillators only work with a limited supply range. Moreover, an oscillator with a frequency tracking loop provides a stable output clock under wide supply voltage and temperature variations, but it requires a bandgap circuit with high precision, which increases the cost.
Therefore, a relaxation oscillator with adaptive supply voltage generation is desired to achieve the best frequency stability and fixed output frequency under a wide supply variation. To accomplish the adaptive supply voltage generation, the present invention proposes a comparator circuit with threshold tracking technique.